The present invention relates to an imaging apparatus which has an image pick-up device, such as a CCD, built in, and more specifically to an image apparatus which permits the device sensitivity to be increased by adding together signal charges produced from pixels in the device.
The present invention also relates to an imaging apparatus which uses a color image pick-up device having a color filter of Bayer arrangement, and more specifically to an image apparatus which permits the device sensitivity to be increased by adding together signal charges produced by pixels in the device.
In recent years, digital still cameras as an electronic imaging apparatuses (i.e., electronic cameras) have been intensively developed which are adapted to capture the image of a subject on the built-in image pick-up device (hereinafter referred to as the imaging device), such as a CCD, and produce a corresponding still or video image signal to reproduce the image. In recent digital cameras, the number of pixels are so increased as to have an improved image quality, but it is still required that shots be continuously taken at short time intervals. Accordingly, it is necessary to improve a circuit system of processing an image so as to satisfy the above requirement. That is, there is required an improved method of reading an image formed on an imaging device having an increased number of pixels at a high speed-reading rate and a high frame transfer rate. To achieve the requirement, in such imaging apparatuses, for example, in still-image or moving-image apparatuses, it is said that, though image resolution is reduced, the sensitivity of the imaging device can be increased by adding together signal charges from adjacent pixels of same color, for example, a total of four pixels: two pixels of same color adjacent to each other in the vertical direction and two pixels of same color adjacent to each other in the horizontal direction.
Specific methods for adding together signal charges include external digital addition by which each signal charge, i.e., each pixel signal, is sequentially read from the imaging device in accordance with standard techniques and then subjected to analog-to-digital conversion and the resulting digital pixel signals are added together in digital circuitry and internal analog addition by which analog signal charges are added together as they are on transfer paths in the CCD imaging device. The internal analog addition requires some modifications to be made to the transfer-driving scheme of the imaging device.
These techniques applied to digital cameras may possibly result in an increase in the sensitivity of the imaging device. However, noise is produced at the time of addition, increasing noise in the image signal. This may cause degradation in image quality in addition to degradation in resolution due to a reduced number of effective pixels.
Some conventional publications describe that, when signal charges from four pixels are added together, the sensitivity of the imaging device is apparently increased by a factor of four. That is not necessarily correct in view of noise produced in the imaging device. The reason is as follows: The effect of improving the SN ratio by adding together signal charges is known to be proportional to the square root of the number of signals to be added together from statistical properties of random noise. Although the signal level is increased by four times by adding together signals from four pixels, the SN ratio can only be expected to increase by a factor of two (i.e., 6 dB). When shooting is made with the image quality, i.e., the noise level, kept, therefore, the sensitivity can only be increased, at a maximum, by a factor of two, in other words, the exposure can only be reduced, at a maximum, by a factor of two. If, to obtain four times the sensitivity, shooting were made at one-quarter the exposure and signals from four pixels ware added together, the signal level S would be increased by a factor of ¼×4=1 and the noise component would be increased by a factor of             1      ×              4              =    2    ,resulting in a reduction of 6 dB in the SN ratio.
Consider here that the imaging device is exposed at one-half the exposure in the case where signals from four pixels are added together. With the external digital addition, each signal from the imaging device is at a level of ½ and hence the noise level is relatively increased by a factor of two. Analog (random) noise is reduced by a factor of       1    /          4        =  2by the subsequent addition of signals from four pixels. There is therefore no problem with respect to the random noise. On the other hand, quantization noise is not reduced even by the signal addition and remains at a relative level of two, causing degradation in image quality as described previously. With the external digital addition, the frame rate remains unchanged from that in normal shooting mode and cannot be increased and moreover digital operations must be performed in two dimensions, increasing the signal processing time for recording.
On the other hand, the analog addition processing involves a problem of saturation of charges within the imaging device, because the saturated charge amount is limited at predetermined values in the charge transfer path and an output section of the imaging device. That is, the maximum output level of the horizontal transfer path on which signal charges are added together or the output amplifiers of the imaging device in the normal read operation is limited to the amount of charge for one pixel. However, the addition of signals from four pixels at one-half the exposure requires the amount of charge for two (=½×4) pixels to be handled. This increase in the amount of charge cannot be accommodated by the imaging device and hence signals will be clipped within the imaging device, causing the image quality to suffer.
Furthermore, since the addition of pixel charges within the imaging device is allowed for only signals from successive pixels, it is said that the application of internal addition processing to the one charged coupled device having a Bayer-arrangement color filter is difficult. The one charged coupled device have therefore to depend on a technique which involves reading pixel signals once, then adding together digital signals of the same color with a digital process outside the imaging device. The addition of pixel signal with external digital processing is complex and takes long because the operations must be performed two-dimensionally and does not allow for an improvement in the frame rate.